thomasdavis

Update: If you had trouble installing the plugin, please download the plugin zip from <a href="http://thomasalwyndavis.com/2010/09/socialsidebar-wordpress-plugin/" rel="nofollow ugc">here</a>
The plugin is custimizable and lets you change the icons if you have fancier ones.
For a more details and a screenshot view the site below
<a href="http://thomasalwyndavis.com/2010/09/socialsidebar-wordpress-plugin/" rel="nofollow ugc">Plugin Site</a>
An example of the plugin is on the right hand side of the page.

Social Sidebar

A popout menu for your website, simple to install and setup, shows social networking icons in a un-obtrusive way.

jcvangent

inboundrocket

Selection Sharer makes it super easy for your readers to share your content and drive new visitors to your site. With Selection Sharer, any visitor on your website/blog can easily 
highlight any part of your content and share it immediately on Twitter, Facebook, LinkedIn or via email. Ultimately, the more your readers share, the more new traffic your site will get.
When a visitor highlights some text on your WordPress website they will be presented by a Medium like popover menu to share their highlighted content. This plugin supports both desktop (with a pop-over) as well as mobile devices (with a pop-under). This plugin is made possible by the excellent work done by <a href="https://github.com/xdamman/selection-sharer" rel="nofollow ugc">@xdamman</a> and is based on his code.

Selection Sharer by Hans van Gent

Works great!

Medium like popover menu to share on Twitter, Facebook, LinkedIn or by email any text selected on the page.

Social Sidebar vs Selection Sharer by Hans van Gent